1. Field of the Invention
This invention relates to an epitaxial semiconductor substrate, manufacturing method thereof, manufacturing method of a semiconductor device and manufacturing method of a solid-state imaging device.
2. Description of the Related Art
As semiconductor substrates for manufacturing semiconductor devices, CZ substrates grown by CZ (Czochralski) method, MCZ substrates grown by MCZ (magnetic field Czochralski) method, and epitaxial substrates having epitaxial layers made on those substrates are often used generally.
As semiconductor substrates for solid-state imaging devices, epitaxial substrates and MCZ substrates are mainly used to reduce uneven image contrast caused by uneven dopant concentration, i.e., striation. Among them, epitaxial substrates can be made to include a low-resistance region (buried region or low-resistance substrate) under epitaxial layers for forming a device, they are effective for progressing low-voltage driving and low power consumption of solid-state imaging devices. Therefore, their wider use is still expected.
For manufacturing silicon (Si) epitaxial substrates, chemical vapor deposition (CVD) is currently used as a practical method, and four kinds of source gases are mainly used therefor. That is, hydrogen reduction process uses SiCl4, or SiHCl3, and reaction occurring there is expressed as follows.
SiCl4 . . . SiCl4+2H2xe2x86x92Si+4HCl
SiHCl3 . . . SiHCl3+H2xe2x86x92Si+3HCl
Thermal decomposition method uses SiH2Cl2 or SiH4, and reaction occurring there is expressed as follows.
SiH2cl2 . . . SiH2Cl2 Si+2HCl
SiH4 . . . SiH4 Si+2H2 
Among these four kinds of source gases, SiHCl3 is inexpensive, grows fast, and is suitable for growth of a thick epitaxial layer. And it is most used for manufacturing Si epitaxial substrates for solid-state imaging devices.
However, whichever one of those source gases is used, Si epitaxial substrates have a high impurity concentration, especially metal impurity such as heavy metal impurity, which undesirably mixes in during deposition of the epitaxial layer. Therefore, so-called white defects due to a dark current of a solid-state imaging device cannot be reduced sufficiently, and this makes characteristics and the production yield poor.
Possible sources of metal impurities such as heavy metal impurities are stainless steel (SUS) members in a bell jar of an epitaxial growth apparatus and source material gas pipes, among others. It is assumed that, if a source gas contains a chlorine (Cl) gas, for example, it decomposes and produces HCl during growth, this corrodes stainless steel members to produce a chloride of a metal, the metal chloride is captured into the source gas, and the metal impurity is caught into the epitaxial layer. In some cases, HCl gas is intentionally introduced into a bell jar to lightly etch off the surface of a Si substrate prior to epitaxial growth of layers, and this is also a cause of corrosion of stainless steel members.
Therefore, when a Si epitaxial substrate is used to fabricate a solid-state imaging device, some gettering technique is necessary for removing metal impurities. As such gettering technique, there are, for example, intrinsic gettering for precipitating over-saturated oxygen in the Si substrate exclusively within the substrate and using it as the getter sink, and extrinsic gettering for making a polycrystalline Si film or a region doped with high-concentrated phosphorus (P) on the bottom surface of the Si substrate and utilizing a distortion stress caused thereby to make a getter sink. None of them, however, had sufficient ability to remove metal impurities from a Si epitaxial substrate, and could not sufficiently reduce white defects of solid-state imaging devices.
Taking the above matters into account, the Inventor previously proposed a method for manufacturing a Si epitaxial substrate by implanting carbon into one of the surfaces of an Si substrate by a dose amount of 5xc3x971013cm xe2x88x922 or higher and thereafter stacking an Si epitaxial layer thereon (Japanese Patent Laid-Open Publication No. hei 6-338507). According to the method, since a getter sink assumed to be a compound of carbon and oxygen in the substrate can powerfully getter metal impurities, etc. mixed into the epitaxial layer, white defects of solid-state imaging devices could be reduced to ⅕ as compared with Si epitaxial substrates made by using conventional gettering method.
To control impurities (especially metal impurities) mixing into epitaxial layers under growth, conventionally used were (1) a method for observing pits or crystal defects in epitaxial layers after growth, (2) a method for quantitatively measuring heavy metal impurities on the surface of an epitaxial layer or in a substrate bulk by atomic absorption spectrometry, inductively coupled plasma mass spectrometry (ICP-MS), or neutron activation analysis, (3) a method for conducting electric measurement such as lifetime measurement on the entirety of an epitaxial substrate by microwaves, and so on.
Among these methods, control of impurities by microwave lifetime measurement needs no pre-treatment, and gets a result quickly and easily. Therefore, microwave lifetime measurement is used widely. With regard to such, the Applicant also proposed a method for reducing white defects of solid-state imaging devices by using a Si epitaxial substrate having a lifetime whose ratio relative to the lifetime of the Si substrate before deposition of the epitaxial layer is larger than a predetermined value (Japanese Patent Laid-Open Publication No. hei 9-139408).
However, in the Si epitaxial substrate treated by carbon gettering, since the getter sink behaves as a center of electron-hole recombination, there is the problem that the measured lifetime does not reflect the amount of impurities mixing into the epitaxial layer under growth. To date, therefore, instead of measuring the life time of a Si epitaxial substrate treated by carbon gettering, the lifetime was measured from a monitor substrate prepared by forming the epitaxial layer on a Si substrate of the same batch but not treated by carbon gettering, and the result was used to evaluate the quality of the Si epitaxial substrate.
However, even among Si epitaxial substrate made in the same batch, a difference among the substrates is inevitable. Therefore, although there is a correlation to an extent between the lifetime measured from the monitor substrate and white defects of solid-state imaging devices manufactured by using Si epitaxial substrates treated by carbon gettering, the correlation is not satisfactory. It is therefore actually difficult to evaluate white defects of solid-state imaging devices, i.e., degree of impurity contamination of Si epitaxial substrates by heavy metal impurities, for example, and accurately know their acceptabiity from the result of measurement of lifetime using a monitor substrate. Furthermore, a wafer-by-wafer type has become the main current of epitaxial devices made by processing a semiconductor substrate as large as 8 inches or more in diameter, and the degree of impurity contamination varies from one sheet of the semiconductor substrate to another. Therefore, measurement of the lifetime using a monitor substrate has become almost meaningless.
In light of the above there is a strong demand for a technique which enables direct measurement of lifetime of a Si epitaxial substrate itself treated by carbon gettering, and can determine acceptability of the Si epitaxial substrate precisely and quickly.
It is therefore an object of the invention to provide an epitaxial semiconductor substrate and its manufacturing method which enables precise and quick determination of acceptability of the epitaxial semiconductor substrate treated by carbon gettering.
Another object of the invention is to provide a method for manufacturing a semiconductor device capable of precisely and quickly determining acceptability of an epitaxial semiconductor substrate treated by carbon gettering and can manufacture a good semiconductor device with a high yield by using a good epitaxial semiconductor substrate remarkably reduced in impurity contamination by heavy metal impurities, for example.
Another object of the invention is to provide a method for manufacturing a solid-state imaging device capable of precisely and quickly determining acceptability of an epitaxial semiconductor substrate treated by carbon gettering and that can manufacture a good semiconductor device with a high yield by using a good epitaxial semiconductor substrate remarkably reduced in white defects.
According to the first aspect of the invention, there is provided an epitaxial semiconductor substrate having an epitaxial layer in which carbon is ion-implanted along a major surface of a semiconductor substrate, and an epitaxial layer made of a semiconductor is stacked on the major surface of the semiconductor substrate, comprising:
a carbon non-implanted region provided at least in one portion along the major surface of the semiconductor substrate.
According to the second aspect of the invention, there is provided a method for manufacturing an epitaxial semiconductor substrate configured to first ion-implant carbon along a major surface of a semiconductor substrate and thereafter stack an epitaxial layer made of a semiconductor on the major surface of the semiconductor substrate, which includes the step of:
ion implanting carbon along the major surface of the semiconductor substrate while making a carbon non-implanted region at least in one location.
According to the third aspect of the invention, there is provided a method for manufacturing a semiconductor device having an epitaxial semiconductor substrate made by first ion-implanting carbon along a major surface of a semiconductor substrate and thereafter stacking an epitaxial layer made of a semiconductor on the major surface of the semiconductor substrate, which includes the steps of:
ion-implanting carbon along the major surface of the semiconductor substrate while making a carbon non-implanted region at least in one location, then making the epitaxial layer on the major surface of the semiconductor substrate, thereafter measuring recombination lifetime or surface photo voltage of a part of the epitaxial layer located above the carbon non-implanted region, using the result thereof to evaluate acceptability of the epitaxial semiconductor substrate, and manufacturing the semiconductor device by using the epitaxial semiconductor substrate evaluated to be good.
According to the fourth aspect of the invention, there is provided a method for manufacturing a solid-state imaging device having an epitaxial semiconductor substrate made by first ion-implanting carbon along a major surface of a semiconductor substrate and thereafter stacking an epitaxial layer made of a semiconductor on the major surface of the semiconductor substrate, which includes the steps of:
ion-implanting carbon along the major surface of the semiconductor substrate while making a carbon non-implanted region at least in one location, then making the epitaxial layer on the major surface of the semiconductor substrate, thereafter measuring recombination lifetime or surface photo voltage of a part of the epitaxial layer located above the carbon non-implanted region, using the result thereof to evaluate acceptability of the epitaxial semiconductor substrate, and manufacturing the solid-state imaging device by using the epitaxial semiconductor substrate evaluated to be good.
In the present invention, from the viewpoint of obtaining sufficient gettering effect by carbon, the dose amount upon ion implantation of carbon into a major surface of the semiconductor substrate is usually not less than 5xc3x971013cmxe2x88x922, and preferably not less than 5xc3x971013cmxe2x88x922 and not more than 5xc3x971015cm xe2x88x922. Basically, configuration and size of a region of the semiconductor substrate in which carbon is not yet implanted (carbon non-implanted region) can be determined freely as far as it is possible to measure recombination lifetime or surface photo voltage (SPV) in the carbon non-implanted region and the overlying part of the epitaxial layer. However, minimum width of the carbon non-implanted region must be larger at least than the recombination lifetime or mean free path in measurement of the surface photo voltage. Normally, it is not less than the thickness of the semiconductor substrate. The carbon non-implanted region may be as large as one chip for manufacturing a semiconductor device by using the epitaxial semiconductor substrate, for example.
Measurement of the recombination lifetime or the surface photo voltage is most excellent as a method for evaluating heavy metal impurities, for example, mixed in during growth of the epitaxial layer. Measurement of the surface photo voltage is attained by making a charge of the same sign with the majority carrier to adhere onto the surface to be measured, then intermittently irradiating thereon monochromatic light of an energy larger than the band gap energy of the substrate, and measuring changes in barrier height of the surface (xe2x88x92qxcex94V) due to the minority carrier generated thereby and moving toward and accumulating on the surface depletion layer. xe2x88x92qxcex94V is the SPV value. As the SPV method, there are a method of adjusting the amount of the irradiated light to make the SPV value constant (constant SPV method) and a method of measuring the SPV value while making the amount of irradiated light constant in the region exhibiting a linear relation between the SPV value and the amount of irradiated light (linear SPV method). The SPV method, in general, uses the diffusion length (L) of the minority carrier as the scale of cleanness of the substrate. The longer the diffusion length, the cleaner the substrate.
In the present invention, determination of acceptability of the epitaxial semiconductor substrate is attained, when using recombination lifetime measurement, typically by measuring recombination lifetime of the semiconductor substrate in the carbon non-implanted region (Tsub), also measuring the recombination lifetime of the epitaxial layer in the portion above the carbon non-implanted region (Tepi), and evaluating whether or not the ratio of the measured value of the recombination lifetime of the epitaxial layer above the carbon non-implanted region relative to the measured value of the recombination life time of the semiconductor substrate in the carbon non-implanted region (Tepi/Tsub) is larger than a predetermined value, preferably not smaller than ⅓, and more preferably not smaller than ⅔. For measuring the surface photo voltage, especially using linear SPV method, determination of acceptability of the epitaxial semiconductor substrate is attained by measuring diffusion length of the semiconductor substrate in the carbon non-implanted region (Lsub), also measuring the diffusion length of the epitaxial layer in the portion above the carbon pre-implanted region (Lepi), and evaluating whether or not the ratio of the measured value of the diffusion length of the epitaxial layer above the carbon non-implanted region relative to the measured value of the diffusion length of the semiconductor substrate in the carbon non-implanted region (Lepi/Lsub) is larger than a predetermined value, preferably not smaller than ⅓, and more preferably not smaller than ⅔. When using diffusion measurement by SPV method, determination of acceptability need not rely on the ratio of measured values of diffusion length (Lepi/Lsub) but may be based on whether or not Lepi is larger than a predetermined value, suitably not less than 200 xcexcm, and more preferably not less than 400 xcexcm.
In the present invention, the solid-state imaging device may be an amplifying type solid-state imaging device or a CMOS solid-state imaging device instead of the CCD solid-state imaging device. The semiconductor device may be any one of various devices such as bipolar LSI, MOSLSI (such as DRAM) or bipolar CMOSLSI instead of those solid-state imaging devices.
In the present invention having the above-summarized construction, by using a location of the epitaxial layer above the carbon non-implanted region of the semiconductor substrate as the measured region, recombination lifetime or surface photo voltage reflecting the true amount of impurities mixed in during growth of the epitaxial layer can be measured directly from the epitaxial semiconductor substrate treated by carbon gettering, and acceptability of the epitaxial semiconductor substrate can be determined precisely and quickly on the basis of the result of the measurement. Then, by using the epitaxial semiconductor substrate evaluated to be good and remarkably reduced in impurity contamination such as heavy metal impurities to manufacture a semiconductor device such as a solid-state imaging device, the invention can ensure high-yield fabrication of a solid-state imaging device or other semiconductor device with remarkably reduced white defects and a good property.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.
FIG. 1 is a cross-sectional view for explaining a method for manufacturing a Si epitaxial substrate according to the first embodiment of the invention;
FIG. 2 is a cross-sectional view for explaining the method for manufacturing a Si epitaxial substrate according to the first embodiment of the invention;
FIG. 3 is a cross-sectional view for explaining the method for manufacturing a Si epitaxial substrate according to the first embodiment of the invention;
FIG. 4 is a cross-sectional view for explaining the method for manufacturing a Si epitaxial substrate according to the first embodiment of the invention;
FIG. 5 is a cross-sectional view for explaining the method for manufacturing a Si epitaxial substrate according to the first embodiment of the invention;
FIG. 6 is a plan view of an example of the Si epitaxial substrate manufactured by the first embodiment;
FIG. 7 is a cross-sectional view for explaining a method for manufacturing a CCD solid-state imaging device according to the second embodiment of the invention;
FIG. 8 is a cross-sectional view for explaining the method for manufacturing a CCD solid-state imaging device according to the second embodiment of the invention;
FIG. 9 is a cross-sectional view for explaining the method for manufacturing a CCD solid-state imaging device according to the second embodiment of the invention;
FIG. 10 is a cross-sectional view for explaining the method for manufacturing a CCD solid-state imaging device according to the second embodiment of the invention;
FIG. 11 is a cross-sectional view for explaining the method for manufacturing a CCD solid-state imaging device according to the second embodiment of the invention; and
FIG. 12 is a graph showing correlation between recombination lifetime measured on a Si epitaxial substrate in a location above a carbon non-implanted region and white defects of a CCD solid-state imaging device manufactured by using the Si epitaxial substrate.